RCA 6T75 Restoration (16GP4)
 

Picked up the Set in South-Eastern Ohio on Friday, and have just gotten it back home to South-Eastern PA.

Other than some dry rotted insulation, and a heap of dust, the set looks pretty decent for 59 years old.

'As is' Pictures attached. (The modern cartridge fuse holder is my own temporary addition). The cabinet is in reasonably good shape, save the one tear in the veneer. It looks better in person, the water spots are on my lens, not the cabinet!

The front and rear access panes for the HV cage are completely missing, so I'll have to machine some new ones-- but that's for another time...
More to follow!

Alex

That's one of the harder models to come by. Congrats are in order.That cabinet style according to RCA was "Moderne".
I may have a rusty door for your cage.It could be cleaned, and, re painted the original flat balack. If this set has the un painted cage, that will work, too. Pm me if interested. Cagee door free. Shipping charges, and, pro. packing by ship, and, pack is all I want. I'll make nothing on this.I just wish to help.
Bill Cahill

Alex, welcome to the forum and to the wonderful world of vintage TV restoration. Please keep us all posted on you restoration progress. For you electrolytic twist-loc cans, you may want to consider the brand new manufacture cans offered by Antique Electronic Supply. They are not cheap, but a good alternative to restuffing.

I would like to see anything you have on early color TV, as that is my primary interest these days.

And don't hesitate to ask questions and post comments, that what the forum is for. We ARE interested. Have fun!

If you're curious what it might take to restuff those cans, check this thread out: http://www.audiokarma.org/forums/sho...=231914&page=2

Looks like a fun set to work on. Good luck!

That was a great thread, thanks. Got the chassis about half cleaned up last night, I'll post some more pictures once I start recapping.

Transformer Rewire and Twist Lock Restuff
 

Got the chassis relatively clean, top side. And dove into 'the works'.

I had previously heat shrunk some tubing over the leads of the main transformer, but the cotton an rubber was still causing issues, flaking off at the 'mouth' of the transformer and doing it's best to short out. I decided to pull open the transformer, remove the original leads, and solder new leads directly on the windings. Where I couldn't get adequate access to the windings, I cut the original lead as short as possible, and heat shrunk the new connection. It turned out okay! I'll post pictures when I get it back together.

I also took out the first twist lock cap, and began the restuffing process. Boy does it take a lot of heat to wick up solder off the chassis connections! I'm used to working with 0402 SMT parts under a magnifying glass -- so this is quite a change.

When I finally got the cap out, I went the dremel route, and created tabs to pull back. However the 'Pull out the terminals and fiber disc and save it,' part didn't go so hot. Even with a lot of heat, the terminals and disc refused to let go from the innards. Eventually I broke the disk by pulling on it, so I'll have to make a new one out of phenolic. Any other tips for better success??

With the disc out, I finally was able to stab some pliers into the can, reheat, and pull the works out. Ive got the new capacitor assemblies ready to go, as soon as I make a new insulator disc. The process is a ton of work, I can understand why people would just put the new caps under the chassis.

Other than that, I just started replacing some caps and resistors -- almost all that I have checked, even the lower valued ones, are out of tolerance.

Once the power supply and other electrolytics are replaced, and new tubes are in, do you generally find that it is relatively safe to attempt to fire up the set? Or is it always advisable to replace all the paper caps first? I have read that people have success with the 'replace a few and test' method.

Any help appreciated as always,
Alex

Alex, I usually just cut the can off right above the step, this leaves the Phenolic intact, then I drill some very tiny holes near each leg for the + leg of the new cap to go through, tie them all together on top and drill one more hole for the ground wire to pass through, glue the can back on with JB weld.

If the can is Hacksaw accessible you can do this without even removing it from the chassis, you do have to be careful not to bust the Phenolic mounting wafers when you do it that way.

Sometimes you can't get all 3 or 4 new caps inside the can, in that case I just put the remaining one under the chassis connected to the lugs.

Hi Alex,

Seems like a good night for stuffing caps :) I've been using a slight variation of the technique you used. I open up the can and removing the steel ring like you've done. Then, I heat up the can with the wafer and pins in place. A slow, steady twist & pull with pliers on the pins and viola the whole slug comes out. Finally, I unrolled the aluminum and goop to free up the pins. This way the entire base is in great condition.

I like Eric's technique too. A pipe cutter can make quick work of the aluminum can. It's especially nice when the cap is in a cardboard sleeve.

Good luck,
Bob

No, not safe to try now. Especially with those black beuties in there, and, in hv cage.
Not advisable.
Where origninals are 1,00 volt, use at least that high voltage, as they have quite a pulse on them. Cap values on damper, and, yoke circuits are fairly crittical, and, disc caps will not work properly. Especially not on vertical circuit.
Clean hv thoroughly, as there is tendency to arc.
Yokes, and, flybacks are prone to failure in these sets, are as vertical oscilator transformers.
I do with out line caps, and, line resistors as without them set chassis is electrically safer to touch.
They stopped noise such as car generators, and, diathermy machines, which we no longer use.
Bill Cahill

Thanks for the input,

I'll take a stab at the heat and twist method on the next twist lock.

I have access to a wide array of HV capacitors, so getting 1kV 5% Films in just about any value isn't a big problem. I'll ohm out, and hipot test the Flyback and vertical oscillator trans. before firing the set up for the first time.

As for the line caps and line resistor, my work in the power electronics industry tells me that the line caps are there as much for RFI suppression from the set out to the world, as for the world into the set. Without them I imagine you could get some local oscillator noise out onto the line which would interfere with other sets. However, I doubt the single order filter really works any miracles -- modern RFI filters are quite an artform in themselves! I'll put in new Y2 safety grade ceramics. I have an isolation transformer at my disposal as well.

I was replacing caps around the vertical oscillator, and something didn't look quite right --My first schematic change!

Sam's shows the vertical oscillator as a 6J5, where as the RCA chart in the cabinet shows the 6SN7 Dual Version. 1/2 of the 6SN7 is connected exactly as the 6J5 would be, with the other 1/2 pulling duty as the synch amp, where sams shows 1/2 12AU7. The 6SN7 is biased differently than what is shown on the 12AU7, but I guess that is to be expected.

Things get even weirder upstream of that at the DC-Restore/Synch Sep tube, where my chassis bears little resemblance to the Sams schematic. There appear to be both halves of a 12AU7 interconnected up there, where sams shows only 1/2.

I haven't fully traced out that schematic yet, but if I get time, i'll do my best to plot it out.

For what its worth, my Sam's guide for the 6T75 shows chassis KCS47, A; where the actual unit is stamped KCS47AT. Does anybody have this particular schematic? Right now i'm pretty much operating on the 'Blindly replace what is there' technique with no schematic of some sections.

Other than that, a few pics of the re-leaded transformer. Luckily wire is available in all colors under the sun!

Alex

Send me an email address and I will send you the schematic.

Great job on rewiring the transformer!!

Well, here it is a week later, and I really went to town on cap replacement. And, with many thanks to John, I now have a (more or less) accurate schematic of the AT chassis. Incidentally, I have come across several 'missing' components in the IF section. There has been at least one omitted capacitor that should be in parallel with an inductance to form one of the traps -- my guess is that this was not a design change, but an error in the original construction. So far I haven't put it in -- we'll see what falls out.

I'm about 90% done with all the paper caps, I am leaving all the mica's in place at the moment. And, I have one of the 3 electrolytic twist locks bodged back in. For now, I've givin up on restuffing them -- I'll worry about authenticity later -- right now I'm still in the 'new project euphoria' stage, and want to get the set up and running.

Current state of the chassis is shown -- Mylar is my capacitor brand of choice -- but there's some Panasonic, Vishay, and a little SBE in there, too.

Incidentally, how in the world do you guys clean the underside of the chassis? When I get a group of caps removed, I clean what I can of the underside of the metal, but even the wiring is completely coated in 'goo.' I dare not use alcohol to clean it, as I fear it will brittle the aging plastics. This set was picked up in major coal country, -- maybe it just lived a dirtly life.

As the crow flies, this was picked up about 80 miles from the Columbus Oh TV market, and also about 80 miles from Charleston, WV. I'm suprised there was anything on the air in 1950...

If I can keep it up a little while longer, I think I'll be ready to bring it up on a variac next weekend. And then, either great exhilaration or mild depression will result :)

Alex

Alex, which inductor has the missing capacitor? As for cleaning, other than a good dusting with an air compressor, I don't try to clean the underside of chassis. The underside of your chassis looks pretty clean to me.

When I bother cleaning the underside, I use odorless mineral spirits and q-tips.

Great job! Your set looks nice! Like I said in my thread, a lot of those cans are made slightly different at the terminal end,(difference in disc material and the way the metal edge is knurled over) my method was used on mid 60s caps in a few Zeniths and an Admiral round color set. Terminals and fiber disc came right out every time. I'll have to try the cut the can off while on the chassis method myself, if I can get into a set with enough room to do so, hasn't happened yet. The Motorola I did used a very annoying material for the disc and almost all of the terminal was aluminum so I had to bring the leads outside of the cap and hide them.

Wow -- a CTC-4, very impressive! Very nice page.

John, the missing parts Ive identified are as follows, using reference designators from the schematic you gave me for the AT chassis:

C131 -- 47pF in the 1&2 video amplifier at the 4.5MHz trap. (Sorry, just can't force myself to say mega cycle!)

The second/third seem less critical, but:

C220 -- 100pF
C219 -- 0.0015uF both in the cathode circuit of the 1st video amplifier

Alex

Alex, while I am not much for those nFarads, I am a Hertzian kinda guy. :-). Hard to know if those were just omissions, or production changes. RCA seemed to be in constant flux with the design of this set. I would try tuning the trap, and if it won't null at 4.5mhz, I would put the cap back. I would probably try the other missing caps after I got the set working and see what difference (if any) they make. Good luck, thanks for keepin' us posted.

Well -- that's it for the Film Cap replacements -- save the Caps in the Damper circuit, and a few other 1kV parts that I had to order.

Other than that, its on to the electrolytics.

Alex

Anybody have any tips for removing rusty tube shields from their tubes?

I have a few that won't give up the fight...

I've pried tube shields off in the past - of course those were the shields that were made of spring metal that just met or overlapped on one side. If they're solid, I'd try dripping a bit of Liquid Wrench or Maltby's on the tube and letting it sit for a few days. You may lose the markings though.

I push them through with a nut driver over the tip, a little lube might help as mentioned.

How did I miss this thread? Great set and congratulations.

That is the exact same chassis in my TV. I'm really having a lot of problems getting mine going.

http://www.audiokarma.org/forums/sho...d.php?t=226988

Thanks for the tips -- Sansuii, you're way ahead of me in performance at this point!

Well, the set has now consumed 90% of my free time for 2 weeks, but I finished recapping the last 5 or 6 films, and all the electrolytics. I guess this is where the real fun begins...

I haven't removed the CRT or the speaker, nor have I made any long wiring harnesses yet, so I put the chassis back in the set, and brought it up slowly on the poor man's variac -- assorted series light bulbs, while monitoring the 375V Bus, as its the only one readily accessible on the picture tube when the chassis is in.

Once I got it up to full line, there was still not much going on, Not so much as static on the speaker. The 375V bus topped out at about 445VDC, seemingly indicating that there's an awful lot of something not drawing very much current. (incidentally, I have gone through most of the resistors I deemed critical -- the power supply chain seemed to check out reasonably well). I flipped the audio input switch to Phono, and hooked up an IPOD on an RCA cable. Still Nada.

Peering inside, it looked like the 6K6 audio output tube was showing some haze, like it might have gone gassy. So much for NOS. I poked around, but all I could come up with was a 6V6 -- close enough to try. I popped it in, and Voila! Sound. Albeit pretty faint, even at top volume. After some more digging, I did find another 6K6, and put it in. Moderately better results -- for about the next 25 seconds when the 5U4 gave up the ghost.

I can see right away that I should have stocked up about 5X more on all the tubes. So. There it is. No signs of raster, no signs of HV. Either that or the CRT is dead. The only up side is that the CRT filament is definitely not open.

I'd consider bringing the set up on silicon diodes at reduced line voltage to get the proper bus, but the death of the 5U4 seems to indicate something is pretty wrong. Plus, the filament voltages would be low. When I get some more 5U4's, I'll start by digging into the other bus voltages.

When you guys probe on a bench, do you always do it with the CRT attached? I was thinking of jumping out the CRT 'loopback' pins on the power supply, and just pulling the horizonal output tube to start looking at things. Whats your standard procedures?

Guess I'm taking hte weekend off, since new 5u4's wont get here for a few days.

Alex

Alex, it seems odd that you would have 445 volts of B+ if there was a short, did the heaters on the 5U4 burn out?

It's a bit late to point this out now but I never recap an entire set all at once any more, there are too many chances to make a mistake. I usually do the main filter cap first and then try it, then if everything is OK I proceed to the next one.

Well any way lets work on what you have, it might be a good idea to use diodes and a 200 watt dim bulb tester while you check things out, it will keep you from having to replace any more 5U4's if there is a problem.

I'd look closely to make sure you haven't wired any electrolytics in backwards, if you have it'll soon show itself in any case when it vents.

There is a 5" CRT used for testing purposes, 5AXP4 I think it is, they show up on eBay now and then.

You don't have to hook up the CRT at all, you can leave the chassis out of the cabinet with the HV lead inside a glass jar to keep it from arcing, this will allow you to do some voltage checks and concentrate on getting your high voltage working.

I'm not sure what you mean by "loopback" pins? you should be able to just leave the CRT base connector unplugged while you work on it, it's not a series string chassis right?

Might be a long shot , but also check the transformer for shorts between the 5 volts windings and the B+ windings , the 445 volts is a bit interesting .
Could there be a missconnection when you changed the transformer wires ?

I have been working on mine on the bench without CRT. You need to jump across two pins on the yoke connector to feed power to the rest of the set. I'm on vacation next week and will try to work on the set and post results.

Does that mean that you were testing it without the yoke connected?
John

Lot of questions in there! Let me try to answer:

Replacing caps piece meal was my first thought, but I got some advice in the forum to go ahead and recap the whole thing. Its neither here nor there -- The set showed no signs of life before recapping, even though the electrolytics weren't totally shot.

1) No Reverse Polarity electrolytics
2)Yes, it seems unlikely there is a short on B+ with 475V. In fact w/ 355V center tap on the 5u4, the peak open circuit voltage is only 500ish. So there's very little drop going on. Im not sure what went in the 5u4, i'll check tonight.

3)The CRT base connector on this set has 2 pins which do nothing but connect the filter choke to the rectifier when the connector is inserted. Ie. if you do nothing, the power supply is completely disconnected downstream of the rectifier.

4)Tube TV -- I'm a little confused -- the 5V winding for the 5u4 filament IS the b+ winding. Maybe I mis-interpreted what you're asking. In any case, with everything else disconnected, all the transformer windigns show isolation.

5)I'll try to bring the set up w/ a pair of diodes and a variac. However, since the diodes will drop much less voltage than the 5u4, i'll have to keep the line voltage low. This will also lower the 6.3V filament taps. Need I be concerned?

Solid state switch mode power supplies are my main game, so I'm still getting used to the tube side of things...
I'll poke around some this weekend and see what I can see.

Alex

Diodes are a no go. The hv will not work below 100-110v ac input.
The picture tube socket isn't the connector to B+. It's two pins on the yoke socket.
This set should be operating at 387v dc on rectifier under full load at a line voltage of 117 volts ac.
Your present dc is way high... Do you have the yoke plugged in?
Also, disconnecting the speaker plug saves the audio out from dammage because it removes the voltage from both the plate, and, screen. However, the more circuits not functionning, the higher your power supply voltage will be.
Also, that single secion boost filter is important. If open, or, shorted, that kills the vertical, and, damper circuits.
Just completely re-cap it.
And, oh, yes. Those 1kv capacitors on audio out., vertical, yoke, horiz. out., and, damper circuits are very important.
Black beuty caps are no good, either.
Get rid of your line caps, and, resistor if you want real relative safety.
The RF it isolated set from is rarely produced anymore.
Cars now run on alternators. Not generators.
And, who uses a diathermy machine anymore?
No real noticable change in rf in signal without the line caps.
I have alot of experience on these sets, so, if you need my help, please feel free to pm, or, e mail me.
Bill Cahill

Yes, I know the b+ is on the yoke, and yes it was connected. I meant CRT connector as a generic term.

If the power line choke were open, I'd get no voltage on B+ at the CRT.

The entire set is recapped.

What makes you say diodes are a no go? The HV shouldn't care what the line voltage is, as long as the DC voltages come up correct.

If you're making the argument that the powerline is cleaner now than in 1950, I'll disagree -- ask the guy who works switch mode supplies! :) Just try running a flourescent light on the same outlet...

I didn't mean that to be Smart Alec-y :) Not sure if it came off that way!!

Well, you did. That's exactly why I rarely come here.
I offer help, and, I'm treated as if I know nothing.
I wanted to help you. That's all.
Good luck with the set.
B C

I have much to learn from everybody's experience with this set, and I welcome and enjoy reading everybody's comments. Thanks for following as I muddle through this.

The engineer in me systematically responds to questions and suggestions. No cold shoulders intended.

I am geniuinely curiuos to know if there is a reason I shouldn't attempt to diode across the rectifier tube -- However, the suggestion to recap specific sections confuses me, as my last post explains that it has already been done. Paper caps, and black beauties included.

Sorry , my error on the 5V winding .....

I was merely warning about voltages of certain caps. I believe is doing the entire thing first.
The rectifier isn't your problem. The set obviously isn't drawing any current.
I'd start looking for open, or, way off value resistors in B+ Supply, and, to each section of the set. This is a major weak point on these sets.
This set should operate at 387 volts at 117 volts ac.
You said, I think, you were getting 488 volts?
That's clearly NOT a rectifier problem.
TV isn't getting out of surge mode.
Check B+ voltages on all major circuits, such as vert. oscilatro, vert. out. caution, don't try to measure plate of output tube, damper, or, plates of horiz. output, hv rect. tubes.
There is a dangerously high ac component there.
The boost filter negative leade connects to cathode of vertical output tube.
Check the .25A fuse in hv cage to make sure it isn't blown.
The correct value of that fuse is .25A 250volt slo blo.

Yoke failures were common then. If horiz. winding shorts enough your hv will fail.
Did you replace all the paper caps in hv cage, also?
There are 4. Two on yoke, and, two on damper.


I've had alot of resistor failures in these sets. They either open, or, go 100 percent, or, more, off value.
Anything above 20 percent should be replaced.
Bill C

I'm still confused about the jumper then. If you had the yoke connected why did you need a jumper?

John

The jumper is original to the set. It's a disconnect in case yoke is accidentally un-plugged.
That cuts all b+ to set.
iF HIS AC line was open, nothing would work. His tubes are lighting, and, he has B+. The problem is more like the set, or, at least part of it, isn't drawing any current.
I still say resistors are likely at fault.
Bill C